Touch screen technologies can be broadly classified into six main categories: resistive, capacitive, peripheral IR, ultrasound, optical imaging, and surface acoustic/pressure. Among those, only resistive, capacitive and optical imaging technologies can readily support multi-touch touch screen capabilities. Although resistive touch screen technology has been the most commonly used technology for small interactive displays due to its relatively low cost, capacitive touch screen technology has been gaining popularity because of its superior touch sensitivity that allows easy and fluid on-screen navigation at the slightest touch.
Both resistive and capacitive touch screens require layered touch sensitive overlays on the display surface and peripheral driver/controllers to process positioning information that increase manufacturing cost and reduce light transmission and image clarity requiring a corresponding increase in the display light output to compensate. Capacitive touch screens have more complex layered substrate structures that are harder to manufacture, thus making it a far more expensive touch technology to implement. Capacitive touch screen also requires the presence of human fingers or something with similar electrical characteristics. Gloved finger or stylus will not work on a capacitive touch screen.
Optical imaging touch screen typically utilizes the principle of frustrated total internal reflection with edge illuminated light trapped within the face glass, released only with a touch of a finger, and one or more cameras capturing the light escaping the glass to determine finger locations. Such an approach requires significant space between the cameras and the screen in order for it to operate properly, making it unsuitable for portable display devices where space is a premium. Other approaches such as ultrasound are even more expensive to implement or are unable to be hermetically sealed to block out dust and moisture, making them generally unsuitable for mobile device applications.
The two main touch screen technologies, resistive and capacitive, have additional drawbacks. Both have relatively low touch resolution in real world implementations as the cost of manufacturing goes up sharply with resolution and concomitantly the touch sensitivity drops precipitously since the sensitivity is directly proportional to the physical dimensions of the individual touch sensors. A more serious issue with both touch technologies is the fact that the position of an object or objects can only be determined after such object make physical contact with the touch screen.
Such deficiencies make many of the gesture-based operations awkward to execute. For example, typing on a virtual keyboard on the touch screen requires the finger to be directly on top of the virtual key to be pressed. Unless the touch screen is large, the virtual key is usually far smaller than the finger itself, hence hitting the right key can be difficult. Such a user interface could be improved if the touch screen can provide a blown up view of the virtual keys underneath the finger and/or the finger itself before the finger touches the screen. Such an interface would require, however, detection of the user's finger before it touches the screen.
Other gestures, such as pinch, zoom, and swipe, also require continuous physical contact between the finger or fingers and the touch screen in order for the touch screen to track the motion of the fingers. This creates wear and tear on the physical screen of devices, degrading the performance of the screen with time. It also makes it hard for the device to ascertain the intention of the user if the latter fails to maintain the physical contact continuously during the gesturing period.